I've discussed this with the Thunderpole guys, and with other CB radio suppliers, and nobody seems to have an answer. It's a real stumper.

New T-1000 CB installation in a car. SWR is highly unstable on all channels. I can change it from 1.0 to > 5 simply by blowing on the cable. I mean that literally -- a movement of the coax by even a couple of millimetres sends the SWR needle from one end of the scale to the other. Just moving around in the car has a similar effect. It's deeply frustrating because when I think I've got it right, I move in my seat, and the SWR is back in the red.

Despite what people keep telling me, I'm almost certain this isn't a grounding or groundplane problem. The antenna mount is connected by a 10" length of fat cable to one of the rear tailgate bolts. The electrical resistance from the mount to the battery negative is zero when nothing but that ground is connected. The car roof is steel and of ordinary size and shape. There are no shorts or opens that I can detect with a meter. I have replaced every part of the system except the radio itself.

Amazingly, the radio does work, even with SWR well into the red, although the range isn't as good as I'd hoped. The fact that it works suggests to me that there are no obvious faults -- antenna insulating stud upside down, that sort of thing -- and in any event these would show up on a meter.

Has anybody seen anything like this before? Or even remotely like it? It's driving me round the bend.

Have you checked antenna mount metal to body work on vehicle,make sure DISCONNECT the PL-259 Plug from radio when carry out this test!!!,check ohm continuity?
Ensure you not just getting a ground via the DC negative lead only,check that outer part of shield on PL Plug is grounded onto vehicle body by touching exposed metal parts\screws on car.

Is whip of antenna touching any part of vehicle body or metal parts or can do when car in motion?

Check PL259 on cable bad joints,poor soldering,small bit of braid shorting PL plug out or if in doubt replace.
If you can try a new patch cable.

IF your antenna mount is a SO-239 type,plug a PL-50ohm dummy load into antenna mount,wiggle cables leads see if VSWR jumps about,handy to have 2 people to test this

Double check the SO-239 socket on radio ,that the locking ring is tight,Ive had some CB radios where these have been lose.
IF you confident,open radio covers up and check centre pin of SO239 socket on radio the pin is not loose,bad solder joint,bad solder joint to Internal PCB,ive seen this too even on new radios.

Blowing on cable a new thing to me in all my 35 years in Radio Communications!